The general requirements for x-ray detectors are well known. In general, some scintillating material is employed which reacts to impinging x-ray photons to create optical photons. In some fashion, the optical photons are typically conducted to an opto-electronic transducer such a photomultiplier or a photodiode, where the optical photons are converted to electrical energy which can be responded to by other, well known electronic components.
Typical x-ray intensities employed in the medical field are on the order of 100 Kev. The present invention is directed at applications requiring substantially higher x-ray intensities, for example 1 Mev or more. The substantially higher x-ray intensities result in a number of problems that have to be overcome which are not met or at least easily overcome in the lower x-ray intensity employed in the medical field.
In the first place, the much higher x-ray intensity requires a detector with increased stopping power. For example, if we were to use a detector suitable for about 100 Kev (such as those used in the medical field) imaging, the much higher (1 Mev or more) x-rays would simply pass through the detector with little or no effect. It is also well known that stopping power can be increased by increasing the thickness of the detector along the direction of the x-ray beam. However, when we increase the thickness of the detector, we want to do it in such a way as not to degrade resolution.
A second problem is the location of the opto-electronic transducers (whether photomultipliers, photodiodes or the like). In order to perform their function, it is essential that the optical photons created in the scintillating material be coupled to the opto-electronic transducers. Transferring the optical-photons from the scintillating material to a light coupler and then to the opto-electronic transducer requires at least two interfaces, and optical photons are lost at each of the interfaces, degrading the signal eventually produced by the opto-electronic transducer. Furthermore, in deciding where to locate the opto-electronic transducers, account must also be taken of the damage that can be done to such transducers by exposure to the x-ray energy. Accordingly, the two problems in this area relate how the optical photons are to be coupled to the opto-electronic transducer and secondly, how the opto-electronic transducers can be located without subjecting them to physical degradation caused by exposure to the x-ray energy.
Furthermore, while we desire to increase the thickness of the scintillating material (so as to provide for sufficient stopping power to detect the impinging x-ray photons) since we are dealing with real materials, we cannot simply increase one dimension, without corresponding increases in other dimensions. Increasing the size of the scintillating material in these other dimensions leads to the possibility that optical photons created in the scintillating material may not reach the location at which they are coupled to the opto-electronic transducer, thus leading to the possibility that the electronic signals will depend in part on the location of the impinging x-ray energy on the scintillating material, giving rise to signal artifacts which are undesirable.
Finally, still another problem to be overcome is the wide dynamic range which is required of the opto-electronic transducers. In many applications in x-ray imaging, x-ray energy of widely varying intensity must be accurately responded to by the detector. For example, relatively weak x-ray intensity can be expected when the object which is being imaged attenuates the x-ray beam. However, it is also typical, in x-ray imaging, to attempt to measure the intensity of the x-ray beam when it is not being attenuated by the object, so as to provide a reference level from which to gauge the attenuation imposed by the object being imaged. Thus another problem to be overcome is providing the detector with sufficient dynamic range so as to accurately measure the unattenuated x-ray intensity as well as the x-ray intensity when it is being attenuated by the object being imaged.